The long term goal is to compare nociceptive processing by excitatory amino acids (EAAs) and substance P (SP) in the spinal cord. The proposed experiments will test the hypothesis that EAAs & SP in the spinal cord are contained in and released from distinct groups of primary afferents, cause distinct nociceptive characteristics which are inhibited by compounds acting at unique receptor subtypes. Sp. Aim 1. Anatomical double labeling procedures will be utilized to determine whether anterogradely labeled primary afferent terminal contain glutamate (glu)- like immunoreactivity, whether these terminal are surrounded by astrocytic processes containing glu dehydrogenase or glutamine synthetase, and whether glu immunoreactive fibers and terminal co-contain CGRP or SP. Sp. Aim 2. We will use in vivo microdialysis in the spinal cords of conscious, freely moving rats to study the release of presumably nociceptive [EEA & SP] and antinociceptive [5HT & NE] transmitters released in response to formalin-injection and in response to specific opioid receptor activity. We will then administer fluorogold, a retrograde tracer, directly into the cannula for combined retrograde tracing-immunocytochemical procedures to identify neurons that may contribute to release. Sp. Aim 3. Microdialysis will be used in anesthetized rats to compare release of EAAs & SP after electrical stimulation of primary afferents at intensities that activate large and large plus small diameter fibers. Sp. Aim 4. Using in vivo microdialysis in spinal cords of freely moving animals together with pharmacologic manipulation of NE, 5HT, SP and EAA activity, we will determine whether EAAs & SP differentially activate distinct descending pathways and whether activity at specific monoamine receptor subtypes differentially alters SP or EAA release. Sp. Aim 5. We will measure autonomic and nociceptive responses in acute and chronic models of allodynia to examine the involvement of EAAs, SP and glycine (gly) in this model of pain. We will compare transmitter release in rats with allodynia to release after formalin-injection (Aim 2). Sp. Aim 6. EAAs, gly & strychnine will be iontophoresed onto antidromically identified nociceptive dorsal horn neurons and determine the effect of gly on NMDA- and SP-induced depolarization in control rats and in formalin & allodynia-models of chronic pain to determine whether gly potentiates NMDA in nociceptive pathways and whether the interaction between gly & NMDA is different in these 2 models of chronic pain.